A steel material used for automobiles, structural materials, and the like is required to be excellent in such mechanical properties as strength, workability, and toughness. In order to improve these mechanical properties comprehensively, it is effective to refine the structure of the steel material. To this end, a number of manufacturing methods for obtaining a steel material with a fine structure have been sought. Further, by refining the structure, it is possible to obtain a high-strength hot-rolled steel sheet having excellent mechanical properties even if the amount of alloy elements added is reduced.
As a method for refining the structure of a steel material, it is known that a large rolling reduction is carried out especially in the later stage of a row of hot finishing mills to refine austenite grains and to increase rolling strains in a rolled material, thereby obtaining fine ferrite grains after rolling. Further, in view of inhibiting recrystallization and recovery of the austenite grains and facilitating the ferrite transformation, it is effective to cool the rolled material to a temperature from 600° C. to 750° C. as quickly as possible after rolling. That is, subsequent to hot finish rolling, it is effective to arrange a cooling device capable of cooling more quickly than ever before to thereby rapidly cool the rolled material after the rolling. And in rapidly cooling the post-rolled material in this way, it is effective to increase a volume of cooling water per unit area sprayed over the steel sheet, that is, to increase a water flow density in order to enhance a cooling capability. And this cooling method is hereinafter called immediate rapid-cooling.
Now, in manufacturing a hot-rolled steel sheet, it is important to control a temperature of a rolled material as accurately as possible so that a desired quality thereof can be finally obtained. For that purpose, a temperature measuring device is disposed in each location in a manufacturing apparatus of a hot-rolled steel sheet in order to know the temperature of the rolled material at each of those locations. In this viewpoint, a temperature measuring device is also disposed on an exit side of a row of hot finishing mills; thereby the temperature of the rolled material at a time of completion of finish rolling can be obtained. Since there is a cooling step afterwards, the temperature on the exit side of the finishing mill is important in determining a final target temperature and a degree of cooling needed to attain this.
However, when rapid cooling by water is performed immediately after finish rolling as described above, it is impossible to measure the temperature of the rolled material on the exit side of the finishing mill. To solve such a problem, Patent Document 1, for example, discloses a means to obtaining the temperature on the exit side of the rolling mill. Patent Document 1 describes that conventionally it has been impossible, because of cooling water, to measure the temperature on the exit side of the rolling mill by using a thermometer disposed on an exit side of a final stand in a row of hot finishing mills, and therefore the temperature of the rolled material is measured on an entry side of the final stand in the row of hot finishing mills.